Hans- would you also want to be notified if someone changes their mind and takes away a best answer from you and gives it to someone else? I don't think of best answers as some sort of certificate to be proud of- I see the feature as being something of use to others who have a similar query, leading them to check the thread, and they can focus first on the Best Answer if they wish so as to save them reading all the others answers.
The Best Answers tab on your profile will tell you all your current best answers, and if you want to make sure you have a record of them before anyone changes their mind you could check them regularly during the day and print off any best answers for your records.